Bridge Loading

[sgs114]

Hello,

We are working on designing a bridge for a maintenance road where the only traffic would be snow-mobiles or a gator w/ trailer to tow equipment. We do not typically do bridges, so I am trying to determine how I can classify the bridge and determine the applicable live load required. Are there AASHTO guidelines that help you categorize a bridges? It seems like H-10 or H-5 loading should be applicable, but I am looking for a reference that indicates the required loading.

SGS

 

[BridgeSmith]

If it's a privately-owned bridge, the loading criteria would be set by the owner. If it's on a public road, the state would likely have a say, and at the very least would require a posting of the load capacity. I would consider it irresponsible to not make it sufficient for all vehicles that could access it, so if it's going to only be designed for certain lightweight vehicles, I would ensure that there are barriers that preclude larger vehicles from getting on it. Even then, I would want load posting signs put up reflecting the load rating per the state's posting guidelines.

 

[TehMightyEngineer]

I'd make it support at least 50% of the H-20 load (H-10 if you prefer). Any less and you might not get a firetruck over it safely.

I've done a lot of private single span precast bridges and unless the owner has a tight budget or other specific constraints I almost always just design it for the full HL-93 vehicle load. For short residential spans the added cost for the higher load is generally small and then you avoid any and all hassles about load ratings and postings and such.

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL) Structural Engineer (IL, HI)

 

[sgs114]

Could either of you recommend a decking material? The location is fairly remote, so access is not super easy. Per the project specs, we are only to use un-treated lumber, steel, or concrete. We have a 30' span and 15' wide bridge. I was thinking cedar planks, but I don't think those would be able to support the H10 loading (we decided to design to this). Alternatively I was thinking pre-cast concrete planks may be our best bet. Any other recommendations I should look into?

SGS

 

[BridgeSmith]

We see a lot of corrugated steel decking (4" to 6") for similar applications. The cedar planks will work, too, if the stringers are spaced closely enough.

Precast, prestressed tri-deck sections (Example) would easily carry anything that could be driven on it. 3 of those would do the job.

There's even precast slab sections (Example), for spans up to 30' (with highway design loads). Drop 4 of those in, and you're good to go.

There's also hollow core slabs, like what's commonly used for parking garages, that could be made more than adequate for an H-10 loading.

 

[SlideRuleEra]

SGS - Consider steel bridge deck grating, as mentioned by Hotrod.

Longer life than untreated wood.
No deck drainage needed.
For a difficult site, lighter (per square foot) than concrete, probably:
1) Making transport to job site easier.
2) Requiring smaller on-site equipment (crane) to install.
3) Lower dead load for substructure to support.

http://www.SlideRuleEra.net